The Role of N-Glycosylation in Function and Surface Trafficking of the Human Dopamine Transporter

Neuromelanin and selective neuronal vulnerability to Parkinson's disease

Neuromelanin is a unique pigment made by some human catecholamine neurons. These neurons survive with their neuromelanin content for a lifetime but can also be affected by age-related neurodegenerative conditions, as observed using new neuromelanin imaging techniques. The limited quantities of neuromelanin has made understanding its normal biology difficult, but recent rodent and primate models, as well as omics studies, have confirmed its importance for selective neuronal loss in Parkinson's disease (PD). We review the development of neuromelanin in dopamine versus noradrenaline neurons and focus on previously overlooked cellular organelles in neuromelanin formation and function. We discuss the role of neuromelanin in stimulating endogenous α-synuclein misfolding in PD which renders neuromelanin granules vulnerable, and can exacerbates other pathogenic processes.

N-acetyl-l-leucine lowers pS129-synuclein and improves synaptic function in models of Parkinson's disease

N-acetyl-L-leucine (NALL), a derivative of the branched-chain amino acid leucine, has shown therapeutic potential in neurodegenerative diseases, including in prodromal stages of Parkinson's disease (PD). However, the mechanism of its protective effects has been largely unknown. Using discovery-based proteomics, we found that treatment with NALL led to upregulation of lysosomal, mitochondrial, and synaptic proteins in PD patient-derived dopaminergic neurons. NALL reduced levels of pathological pS129-alpha-synuclein in dopaminergic neurons from patients harboring GBA1 or LRRK2 mutations. This decrease in pS129-syn was dependent on serine protease HTRA1 that was induced by NALL treatment of dopaminergic neurons. NALL also upregulated expression of wild-type parkin in both GBA1 and LRRK2 mutant neurons, leading to an increase in functional dopamine transporter and synaptic membrane-associated synaptojanin-1, suggesting improved synaptic function. Furthermore, NALL treatment of mutant LRRK2R1441C knock-in mice led to decreased pS129-alpha-synuclein, increased parkin and improved dopamine-dependent motor learning deficits. These findings highlight the therapeutic potential of NALL in PD by its protective effects on α-synuclein pathology and synaptic function in vulnerable dopaminergic neurons.

A drug repurposing screen reveals dopamine signaling as a critical pathway underlying potential therapeutics for the rare disease DPAGT1-CDG

DPAGT1-CDG is a Congenital Disorder of Glycosylation (CDG) that lacks effective therapies. It is caused by mutations in the gene DPAGT1 which encodes the first enzyme in N-linked glycosylation. We used a Drosophila rough eye model of DPAGT1-CDG with an improperly developed, small eye phenotype. We performed a drug repurposing screen on this model using 1,520 small molecules that are 98% FDA/EMA-approved to find drugs that improved its eye. We identified 42 candidate drugs that improved the DPAGT1-CDG model. Notably from this screen, we found that pharmacological and genetic inhibition of the dopamine D2 receptor partially rescued the DPAGT1-CDG model. Loss of both dopamine synthesis and recycling partially rescued the model, suggesting that dopaminergic flux and subsequent binding to D2 receptors is detrimental under DPAGT1 deficiency. This links dopamine signaling to N-glycosylation and represents a new potential therapeutic target for treating DPAGT1-CDG. We also genetically validate other top drug categories including acetylcholine-related drugs, COX inhibitors, and an inhibitor of NKCC1. These drugs and subsequent analyses reveal novel biology in DPAGT1 mechanisms, and they may represent new therapeutic options for DPAGT1-CDG.

Comparative analysis of uninduced and neuronally-induced human dental pulp stromal cells in a 6-OHDA model of Parkinson's disease

BACKGROUND

In recent years, dental pulp stromal cells (DPSCs) have emerged as a promising therapeutic approach for Parkinson's disease (PD), owing to their inherent neurogenic potential and the lack of neuroprotective treatments for this condition. However, uncertainties persist regarding the efficacy of these cells in an undifferentiated state versus a neuronally-induced state. This study aims to delineate the distinct therapeutic potential of uninduced and neuronally-induced DPSCs in a rodent model of PD induced by 6-Hydroxydopamine (6-OHDA).

METHODS

DPSCs were isolated from human teeth, characterized as mesenchymal stromal cells, and induced to neuronal differentiation. Neuronal markers were assessed before and after induction. DPSCs were transplanted into the substantia nigra pars compacta (SNpc) of rats 7 days following the 6-OHDA lesion. In vivo tracking of the cells, evaluation of locomotor behavior, dopaminergic neuron survival, and the expression of essential proteins within the dopaminergic system were conducted 7 days postgrafting.

RESULTS

Isolated DPSCs exhibited typical characteristics of mesenchymal stromal cells and maintained a normal karyotype. DPSCs consistently expressed neuronal markers, exhibiting elevated expression of βIII-tubulin following neuronal induction. Results from the animal model showed that both DPSC types promoted substantial recovery in dopaminergic neurons, correlating with enhanced locomotion. Additionally, neuronally-induced DPSCs prevented GFAP elevation, while altering DARPP-32 phosphorylation states. Conversely, uninduced DPSCs reduced JUN levels. Both DPSC types mitigated the elevation of glycosylated DAT.

CONCLUSIONS

Our results suggested that uninduced DPSCs and neuronally-induced DPSCs exhibit potential in reducing dopaminergic neuron loss and improving locomotor behavior, but their underlying mechanisms differ.

Parkinson's disease gene, Synaptojanin1, dysregulates the surface maintenance of the dopamine transporter

Missense mutations of PARK20/SYNJ1 (synaptojanin1/Synj1) were found in complex forms of familial Parkinsonism. However, the Synj1-regulated molecular and cellular changes associated with dopaminergic dysfunction remain unknown. We now report a fast depletion of evoked dopamine and impaired maintenance of the axonal dopamine transporter (DAT) in the Synj1 haploinsufficient (Synj1+/-) neurons. While Synj1 has been traditionally known to facilitate the endocytosis of synaptic vesicles, we provide in vitro and in vivo evidence demonstrating that Synj1 haploinsufficiency results in an increase of total DAT but a reduction of the surface DAT. Synj1+/- neurons exhibit maladaptive DAT trafficking, which could contribute to the altered DA release. We showed that the loss of surface DAT is associated with the impaired 5'-phosphatase activity and the hyperactive PI(4,5)P2-PKCβ pathway downstream of Synj1 deficiency. Thus, our findings provided important mechanistic insight for Synj1-regulated DAT trafficking integral to dysfunctional DA signaling, which might be relevant to early Parkinsonism.

Structure of the human dopamine transporter in complex with cocaine

The dopamine transporter (DAT) is crucial for regulating dopamine signalling and is the prime mediator for the rewarding and addictive effects of cocaine1. As part of the neurotransmitter sodium symporter family, DAT uses the Na+ gradient across cell membranes to transport dopamine against its chemical gradient2. The transport mechanism involves both intra- and extracellular gates that control substrate access to a central site. However, the molecular intricacies of this process and the inhibitory mechanism of cocaine have remained unclear. Here, we present the molecular structure of human DAT in complex with cocaine at a resolution of 2.66 Å. Our findings reveal that DAT adopts the expected LeuT-fold, posing in an outward-open conformation with cocaine bound at the central (S1) site. Notably, while an Na+ occupies the second Na+ site (Na2), the Na1 site seems to be vacant, with the side chain of Asn82 occupying the presumed Na+ space. This structural insight elucidates the mechanism for the cocaine inhibition of human DAT and deepens our understanding of neurotransmitter transport. By shedding light on the molecular underpinnings of how cocaine acts, our study lays a foundation for the development of targeted medications to combat addiction.

Structure of the human dopamine transporter and mechanisms of inhibition

The neurotransmitter dopamine has central roles in mood, appetite, arousal and movement1. Despite its importance in brain physiology and function, and as a target for illicit and therapeutic drugs, the human dopamine transporter (hDAT) and mechanisms by which it is inhibited by small molecules and Zn2+ are without a high-resolution structural context. Here we determine the structure of hDAT in a tripartite complex with the competitive inhibitor and cocaine analogue, (-)-2-β-carbomethoxy-3-β-(4-fluorophenyl)tropane2 (β-CFT), the non-competitive inhibitor MRS72923 and Zn2+ (ref. 4). We show how β-CFT occupies the central site, approximately halfway across the membrane, stabilizing the transporter in an outward-open conformation. MRS7292 binds to a structurally uncharacterized allosteric site, adjacent to the extracellular vestibule, sequestered underneath the extracellular loop 4 (EL4) and adjacent to transmembrane helix 1b (TM1b), acting as a wedge, precluding movement of TM1b and closure of the extracellular gate. A Zn2+ ion further stabilizes the outward-facing conformation by coupling EL4 to EL2, TM7 and TM8, thus providing specific insights into how Zn2+ restrains the movement of EL4 relative to EL2 and inhibits transport activity.

Dopamine reuptake and inhibitory mechanisms in human dopamine transporter

The dopamine transporter has a crucial role in regulation of dopaminergic neurotransmission by uptake of dopamine into neurons and contributes to the abuse potential of psychomotor stimulants1-3. Despite decades of study, the structure, substrate binding, conformational transitions and drug-binding poses of human dopamine transporter remain unknown. Here we report structures of the human dopamine transporter in its apo state, and in complex with the substrate dopamine, the attention deficit hyperactivity disorder drug methylphenidate, and the dopamine-uptake inhibitors GBR12909 and benztropine. The dopamine-bound structure in the occluded state precisely illustrates the binding position of dopamine and associated ions. The structures bound to drugs are captured in outward-facing or inward-facing states, illuminating distinct binding modes and conformational transitions during substrate transport. Unlike the outward-facing state, which is stabilized by cocaine, GBR12909 and benztropine stabilize the dopamine transporter in the inward-facing state, revealing previously unseen drug-binding poses and providing insights into how they counteract the effects of cocaine. This study establishes a framework for understanding the functioning of the human dopamine transporter and developing therapeutic interventions for dopamine transporter-related disorders and cocaine addiction.

Kappa Opioid Receptor Antagonism Restores Phosphorylation, Trafficking and Behavior induced by a Disease Associated Dopamine Transporter Variant

Aberrant dopamine (DA) signaling is implicated in schizophrenia, bipolar disorder (BPD), autism spectrum disorder (ASD), substance use disorder, and attention-deficit/hyperactivity disorder (ADHD). Treatment of these disorders remains inadequate, as exemplified by the therapeutic use of d-amphetamine and methylphenidate for the treatment of ADHD, agents with high abuse liability. In search for an improved and non-addictive therapeutic approach for the treatment of DA-linked disorders, we utilized a preclinical mouse model expressing the human DA transporter (DAT) coding variant DAT Val559, previously identified in individuals with ADHD, ASD, or BPD. DAT Val559, like several other disease-associated variants of DAT, exhibits anomalous DA efflux (ADE) that can be blocked by d-amphetamine and methylphenidate. Kappa opioid receptors (KORs) are expressed by DA neurons and modulate DA release and clearance, suggesting that targeting KORs might also provide an alternative approach to normalizing DA-signaling disrupted by perturbed DAT function. Here we demonstrate that KOR stimulation leads to enhanced surface trafficking and phosphorylation of Thr53 in wildtype DAT, effects achieved constitutively by the Val559 mutant. Moreover, these effects can be rescued by KOR antagonism of DAT Val559 in ex vivo preparations. Importantly, KOR antagonism also corrected in vivo DA release as well as sex-dependent behavioral abnormalities observed in DAT Val559 mice. Given their low abuse liability, our studies with a construct valid model of human DA associated disorders reinforce considerations of KOR antagonism as a pharmacological strategy to treat DA associated brain disorders.

A Conserved Intramolecular Ion-Pair Plays a Critical but Divergent Role in Regulation of Dimerization and Transport Function among the Monoamine Transporters

The monoamine transporters, including the serotonin transporter (SERT), dopamine transporter (DAT), and norepinephrine transporter (NET), are the therapeutic targets for the treatment of many neuropsychiatric disorders. Despite significant progress in characterizing the structures and transport mechanisms of these transporters, the regulation of their transport functions through dimerization or oligomerization remains to be understood. In the present study, we identified a conserved intramolecular ion-pair at the third extracellular loop (EL3) connecting TM5 and TM6 that plays a critical but divergent role in the modulation of dimerization and transport functions among the monoamine transporters. The disruption of the ion-pair interactions by mutations induced a significant spontaneous cross-linking of a cysteine mutant of SERT and an increase in cell surface expression but with an impaired specific transport activity. On the other hand, similar mutations of the corresponding ion-pair residues in both DAT and NET resulted in an opposite effect on their oxidation-induced dimerization, cell surface expression, and transport function. Reversible biotinylation experiments indicated that the ion-pair mutations slowed down the internalization of SERT but stimulated the internalization of DAT. In addition, cysteine accessibility measurements for monitoring SERT conformational changes indicated that substitution of the ion-pair residues resulted in profound effects on the rate constants for cysteine modification in both the extracellular and cytoplasmatic substrate permeation pathways. Furthermore, molecular dynamics simulations showed that the ion-pair mutations increased the interfacial interactions in a SERT dimer but decreased it in a DAT dimer. Taken together, we propose that the transport function is modulated by the equilibrium between monomers and dimers on the cell surface, which is regulated by a potential compensatory mechanism but with different molecular solutions among the monoamine transporters. The present study provided new insights into the structural elements regulating the transport function of the monoamine transporters through their dimerization.

Sleep and Pain: A Role for the Anterior Cingulate Cortex, Nucleus Accumbens, and Dopamine in the Increased Pain Sensitivity Following Sleep Restriction

Persistent pain conditions and sleep disorders are public health problems worldwide. It is widely accepted that sleep disruption increases pain sensitivity; however, the underlying mechanisms are poorly understood. In this study, we used a protocol of 6 hours a day of total sleep deprivation for 3 days in rats to advance the understanding of these mechanisms. We focused on gender differences and the dopaminergic mesocorticolimbic system. The findings demonstrated that sleep restriction (SR) increased pain sensitivity in a similar way in males and females, without inducing a significant stress response. This pronociceptive effect depends on a nucleus accumbens (NAc) neuronal ensemble recruited during SR and on the integrity of the anterior cingulate cortex (ACC). Data on indirect dopaminergic parameters, dopamine transporter glycosylation, and dopamine and cyclic adenosine monophosphate (AMP)-regulated phosphoprotein-32 phosphorylation, as well as dopamine, serotonin, and norepinephrine levels, suggest that dopaminergic function decreases in the NAc and ACC after SR. Complementarily, pharmacological activation of dopamine D2, but not D1 receptors either in the ACC or in the NAc prevents SR from increasing pain sensitivity. The ACC and NAc are the main targets of dopaminergic mesocorticolimbic projections with a key role in pain modulation. This study showed their integrative role in the pronociceptive effect of SR, pointing to dopamine D2 receptors as a potential target for pain management in patients with sleep disorders. These findings narrow the focus of future studies on the mechanisms by which sleep impairment increases pain sensitivity. PERSPECTIVE: This study demonstrates that the pronociceptive effect of SR affects similarly males and females and depends on a NAc neuronal ensemble recruited during SR and on the integrity of the ACC. Findings on dopaminergic function support dopamine D2 receptors as targets for pain management in sleep disorders patients.

Endocytic Roles of Glycans on Proteins and Lipids

Most cell surface proteins are decorated by glycans, and the plasma membrane is rich in glycosylated lipids. The mechanisms by which the enormous complexity of these glycan structures on proteins and lipids is exploited to control glycoprotein activity by setting their cell surface residence time and the ways by which they are taken up into cells are still under active investigation. Here, two mechanisms are presented, termed galectin lattices and glycolipid-lectin (GL-Lect)-driven endocytosis, which are among the most prominent to establish a link between glycan information and endocytosis. Types of glycans on glycoproteins and glycolipids are reviewed from the angle of their interaction with glycan-binding proteins that are at the heart of galectin lattices and GL-Lect-driven endocytosis. Examples are given to show how these mechanisms affect cellular functions ranging from cell migration and signaling to vascularization and immune modulation. Finally, outstanding challenges on the link between glycosylation and endocytosis are discussed.

N-Glycans on the extracellular domain of the Notch1 receptor control Jagged-1 induced Notch signalling and myogenic differentiation of S100β resident vascular stem cells

Notch signalling, critical for development and postnatal homeostasis of the vascular system, is highly regulated by several mechanisms including glycosylation. While the importance of O-linked glycosylation is widely accepted, the structure and function of N-glycans has yet to be defined. Here, we take advantage of lectin binding assays in combination with pharmacological, molecular, and site-directed mutagenetic approaches to study N-glycosylation of the Notch1 receptor. We find that several key oligosaccharides containing bisecting or core fucosylated structures decorate the receptor, control expression and receptor trafficking, and dictate Jagged-1 activation of Notch target genes and myogenic differentiation of multipotent S100β vascular stem cells. N-glycans at asparagine (N) 1241 and 1587 protect the receptor from accelerated degradation, while the oligosaccharide at N888 directly affects signal transduction. Conversely, N-linked glycans at N959, N1179, N1489 do not impact canonical signalling but inhibit differentiation. Our work highlights a novel functional role for N-glycans in controlling Notch1 signalling and differentiation of vascular stem cells.

Mass Spectrometric Analysis of Urinary N-Glycosylation Changes in Patients with Parkinson's Disease

Urine is thought to provide earlier and more sensitive molecular changes for biomarker discovery than blood. Numerous glycoproteins, peptides, and free glycans are present in urine through glomerular filtration of plasma, cell shedding, apoptosis, proteolytic cleavage, and exosome secretion. Urine biomarkers have enormous diagnostic potential, and the use of these biomarkers is a long-standing practice. The discovery of non-urological disease biomarkers from urine is also gaining attention due to its non-invasive sample collection and ease of analysis. Abnormal protein glycosylation in plasma or cerebrospinal fluid has been associated with Parkinson's disease, however, whether urine with Parkinson's disease has characteristic glycosylation remains to be explored. Here, we use mass spectrometry-based glycomics and glycoproteomics approaches to analyze urine samples for glycans, glycosites, and intact glycopeptides of urine samples. Reduced abundance of N-glycans was detected at the level of total glycans as well as specific glycosites of glycopeptides. The most abundant N-glycan in urine is S(6)1H5N4F1; S(6)2H5N4 and N4H4F1 are highly present in serum and urine, and 10 biantennary galactosylated N-glycans in the urine of PD patients were significantly decreased. The downregulation of sialylation may be due to the reduction of ST3GAL2. Site-specific N-glycosylation analysis revealed that AMBP, UMOD, and RNase1 have PD-specific N-glycosylation sites. GO and KEGG analysis revealed that N-glycosylation changes may provide clues to identify disease-specific glycosylation biomarkers in Parkinson's disease.

Dessert or Poison? The Roles of Glycosylation in Alzheimer's, Parkinson's, Huntington's Disease, and Amyotrophic Lateral Sclerosis

Ministry of Education and Key Laboratory of Neurons and glial cells of the central nervous system (CNS) are modified by glycosylation and rely on glycosylation to achieve normal neural function. Neurodegenerative disease is a common disease of the elderly, affecting their healthy life span and quality of life, and no effective treatment is currently available. Recent research implies that various glycosylation traits are altered during neurodegenerative diseases, suggesting a potential implication of glycosylation in disease pathology. Herein, we summarized the current knowledge about glycosylation associated with Alzheimer's disease (AD), Parkinson's disease (PD), Huntington's disease (HD), and Amyotrophic lateral sclerosis (ALS) pathogenesis, focusing on their promising functional avenues. Moreover, we collected research aimed at highlighting the need for such studies to provide a wealth of disease-related glycosylation information that will help us better understand the pathophysiological mechanisms and hopefully specific glycosylation information to provide further diagnostic and therapeutic directions for neurodegenerative diseases.

Comprehensive Analysis of Universal Stress Protein Family Genes and Their Expression in Fusarium oxysporum Response of Populus davidiana × P. alba var. pyramidalis Louche Based on the Transcriptome

Universal stress proteins (USPs) are typical stress-inducible proteins that function directly in a variety of biotic or abiotic stresses and effectively protect plants from complex, adverse environments. However, the expression patterns of USP genes under pathogen stress and their molecular mechanisms in stress resistance have not been reported in detail. In this study, 46 USP genes were identified from Populus trichocarpa (PtrUSPs), and their biological characteristics were comprehensively analyzed based on phylogeny, physicochemical properties of proteins, and gene structures. The promoter regions of PtrUSPs contain a variety of cis-acting elements related to hormone and stress response. The results of a collinearity analysis showed that PtsrUSPs were highly conserved with homologous genes from four other representative species (Arabidopsis thaliana, Eucalyptus grandis, Glycine max, and Solanum lycopersicum). Furthermore, RNA-Seq analysis showed that the expression of 46 USPs from P. davidiana × P. alba var. pyramidalis Louche (PdpapUSPs) was significantly induced by Fusarium oxysporum. The co-expression network and gene ontology analysis of PtrUSPs showed that they participated in the response to stress and response to stimulus through precise coordination. The results of this paper systematically revealed the biological characteristics of PtrUSPs and the characteristics of their response to F. oxysporum stress, which will lay a theoretical foundation for improving genetic traits and the breeding of poplar disease-resistant varieties in subsequent studies.

Effect of PACAP/PAC1R on Follicle Development of Djungarian Hamster (Phodopus sungorus) with the Variation of Ambient Temperatures

In Phodopus sungorus, the relationship between pituitary adenylate cyclase-activating polypeptide (PACAP) and its receptor (PAC1R), follicle-stimulating hormone (FSH), and follicle development remains unclear. In this study, we found that the development of growing follicles and antral follicles were inhibited at low (8 °C, 14 °C) and high (29 °C) temperatures. Meanwhile, PACAP/PAC1R expression and follicle-stimulating hormone (FSH) serum concentration significantly decreased during ambient temperatures of 8 °C, 14 °C and 29 °C compared to 21 °C. Thus, ambient temperature may influence the expression of PACAP/PAC1R and the synthesis of FSH for involvement in follicle development. Moreover, PACAP/PAC1R had major functional elements including PKA/PKG and PKC phosphorylation sites, which may involve in the pathway of FSH synthesis through cAMP-PKA and its downstream signal pathway. Moreover, there was a significant positive correlation between the expression levels of PACAP/PAC1R and the number of the growing and antral follicles, as well as the serum FSH concentration and the number of antral follicles. However, there was no significant correlation between the expression levels of PACAP/PAC1R and the serum FSH concentration, indicating a complicated pathway between PACAP/PAC1R and FSH. In conclusion, ambient temperature affects the expression of PACAP/PAC1R and the serum FSH concentration. The expression of PACAP/PAC1R and the serum FSH concentration are correlated with follicle development, which implies that they are involved in follicle development, which will ultimately influence the reproduction of Phodopus sungorus. This study can lay the foundation for future investigation on the regulation mechanism of reproduction in Phodopus sungorus.

Cocaine-regulated trafficking of dopamine transporters in cultured neurons revealed by a pH sensitive reporter

Cocaine acts by inhibiting plasma membrane dopamine transporter (DAT) function and altering its surface expression. The precise manner and mechanism by which cocaine regulates DAT trafficking, especially at neuronal processes, are poorly understood. In this study, we engineered and validated the use of DAT-pHluorin for studying DAT localization and its dynamic trafficking at neuronal processes of cultured mouse midbrain neurons. We demonstrate that unlike neuronal soma and dendrites, which contain a majority of the DATs in weakly acidic intracellular compartments, axonal DATs at both shafts and boutons are primarily (75%) localized to the plasma membrane, whereas large varicosities contain abundant intracellular DAT within acidic intracellular structures. We also demonstrate that cocaine exposure leads to a Synaptojanin1-sensitive DAT internalization process followed by membrane reinsertion that lasts for days. Thus, our study reveals the previously unknown dynamics and molecular regulation for cocaine-regulated DAT trafficking in neuronal processes.

Loss of bisecting GlcNAcylation on MCAM of bone marrow stoma determined pro-tumoral niche in MDS/AML

Bone marrow (BM) stroma plays key roles in supporting hematopoietic stem cell (HSC) growth. Glycosylation contributes to the interactions between HSC and surrounding microenvironment. We observed that bisecting N-acetylglucosamine (GlcNAc) structures, in BM stromal cells were significantly lower for MDS/AML patients than for healthy subjects. Malignant clonal cells delivered exosomal miR-188-5p to recipient stromal cells, where it suppressed bisecting GlcNAc by targeting MGAT3 gene. Proteomic analysis revealed reduced GlcNAc structures and enhanced expression of MCAM, a marker of BM niche. We characterized MCAM as a bisecting GlcNAc-bearing target protein, and identified Asn 56 as bisecting GlcNAc modification site on MCAM. MCAM on stromal cell surface with reduced bisecting GlcNAc bound strongly to CD13 on myeloid cells, activated responding ERK signaling, and thereby promoted myeloid cell growth. Our findings, taken together, suggest a novel mechanism whereby MDS/AML clonal cells generate a self-permissive niche by modifying glycosylation level of stromal cells.

Overview of the structure and function of the dopamine transporter and its protein interactions

The dopamine transporter (DAT) plays an integral role in dopamine neurotransmission through the clearance of dopamine from the extracellular space. Dysregulation of DAT is central to the pathophysiology of numerous neuropsychiatric disorders and as such is an attractive therapeutic target. DAT belongs to the solute carrier family 6 (SLC6) class of Na⁺/Cl^- dependent transporters that move various cargo into neurons against their concentration gradient. This review focuses on DAT (SCL6A3 protein) while extending the narrative to the closely related transporters for serotonin and norepinephrine where needed for comparison or functional relevance. Cloning and site-directed mutagenesis experiments provided early structural knowledge of DAT but our contemporary understanding was achieved through a combination of crystallization of the related bacterial transporter LeuT, homology modeling, and subsequently the crystallization of drosophila DAT. These seminal findings enabled a better understanding of the conformational states involved in the transport of substrate, subsequently aiding state-specific drug design. Post-translational modifications to DAT such as phosphorylation, palmitoylation, ubiquitination also influence the plasma membrane localization and kinetics. Substrates and drugs can interact with multiple sites within DAT including the primary S1 and S2 sites involved in dopamine binding and novel allosteric sites. Major research has centered around the question what determines the substrate and inhibitor selectivity of DAT in comparison to serotonin and norepinephrine transporters. DAT has been implicated in many neurological disorders and may play a role in the pathology of HIV and Parkinson's disease via direct physical interaction with HIV-1 Tat and α-synuclein proteins respectively.

Cross-Linking and Functional Analyses for Dimerization of a Cysteine Mutant of Glycine Transporter 1

Glycine transporter 1 (GlyT1) is responsible for the reuptake of glycine, which regulates glutamate signaling as a co-agonist with N-methyl-D-aspartic acid (NMDA) receptors in the excitatory synapse and has been proposed to be a potential target in the development of therapies for a broad range of disorders of the central nervous system. Despite significant progress in characterizing structure and transport mechanism of the transporter, the regulation of transport function through oligomerization remains to be understood. In the present work, association of two forms of GlyT1 into dimers and higher order oligomers was detected by coimmunoprecipitation. To investigate functional properties of dimers of a GlyT1 cysteine mutant L288C, we performed oxidative cross-linking of the positioned cysteine residues in extracellular loop 3 (EL3) near the extracellular end of TM6. By analyzing the effect of copper phenanthroline (CuP)-induced dimerization on transport function, cross-linking of L288C was found to inhibit transport activity. In addition, an intramolecular ion pair Lys286-Glu289 was revealed to be critical for stabilizing EL3 in a conformation that modulates CuP-induced dimerization and transport function of the GlyT1 L288C mutant. Furthermore, the influence of transporter conformation on GlyT1 L288C dimerization was investigated. The substrate glycine, in the presence of both Na⁺ and Cl^-, significantly reduced oxidative cross-linking, suggesting a large-scale rotation of the bundle domain during substrate transport impairs interfacial interactions between L288C protomers. The present study provides new insights into structural and functional elements regulating GlyT1 transport activity through its dimerization or oligomerization.

3D biocomposite culture enhances differentiation of dopamine-like neurons from SH-SY5Y cells: A model for studying Parkinson's disease phenotypes

Studies of underlying neurodegenerative processes in Parkinson's Disease (PD) have traditionally utilized cell cultures grown on two-dimensional (2D) surfaces. Biomimetic three-dimensional (3D) cell culture platforms have been developed to better emulate features of the brain's natural microenvironment. We here use our bioengineered brain-like tissue model, composed of a silk-hydrogel composite, to study the 3D microenvironment's contributions on the development and performance of dopaminergic-like neurons (DLNs). Compared with 2D culture, SH-SY5Y cells differentiated in 3D microenvironments were enriched for DLNs concomitant with a reduction in proliferative capacity during the neurodevelopmental process. Additionally, the 3D DLN cultures were more sensitive to oxidative stresses elicited by the PD-related neurotoxin 1-methyl-4-phenylpyridinium (MPP). MPP induced transcriptomic profile changes specific to 3D-differentiated DLN cultures, replicating the dysfunction of neuronal signaling pathways and mitochondrial dynamics implicated in PD. Overall, this physiologically-relevant 3D platform resembles a useful tool for studying dopamine neuron biology and interrogating molecular mechanisms underlying neurodegeneration in PD.

Biology of Peptide Transporter 2 in Mammals: New Insights into Its Function, Structure and Regulation

Peptide transporter 2 (PepT2) in mammals plays essential roles in the reabsorption and conservation of peptide-bound amino acids in the kidney and in maintaining neuropeptide homeostasis in the brain. It is also of significant medical and pharmacological significance in the absorption and disposing of peptide-like drugs, including angiotensin-converting enzyme inhibitors, β-lactam antibiotics and antiviral prodrugs. Understanding the structure, function and regulation of PepT2 is of emerging interest in nutrition, medical and pharmacological research. In this review, we provide a comprehensive overview of the structure, substrate preferences and localization of PepT2 in mammals. As PepT2 is expressed in various organs, its function in the liver, kidney, brain, heart, lung and mammary gland has also been addressed. Finally, the regulatory factors that affect the expression and function of PepT2, such as transcriptional activation and posttranslational modification, are also discussed.

Norepinephrine activates β1 -adrenergic receptors at the inner nuclear membrane in astrocytes

Norepinephrine exerts powerful influences on the metabolic, neuroprotective and immunoregulatory functions of astrocytes. Until recently, all effects of norepinephrine were believed to be mediated by receptors localized exclusively to the plasma membrane. However, recent studies in cardiomyocytes have identified adrenergic receptors localized to intracellular membranes, including Golgi and inner nuclear membranes, and have shown that norepinephrine can access these receptors via transporter-mediated uptake. We recently identified a high-capacity norepinephrine transporter, organic cation transporter 3 (OCT3), densely localized to outer nuclear membranes in astrocytes, suggesting that adrenergic signaling may also occur at the inner nuclear membrane in these cells. Here, we used immunofluorescence and western blot to show that β1 -adrenergic receptors are localized to astrocyte inner nuclear membranes; that key adrenergic signaling partners are present in astrocyte nuclei; and that OCT3 and other catecholamine transporters are localized to astrocyte plasma and nuclear membranes. To test the functionality of nuclear membrane β1 -adrenergic receptors, we monitored real-time protein kinase A (PKA) activity in astrocyte nuclei using a fluorescent biosensor. Treatment of astrocytes with norepinephrine induced rapid increases in PKA activity in the nuclear compartment. Pretreatment of astrocytes with inhibitors of catecholamine uptake blocked rapid norepinephrine-induced increases in nuclear PKA activity. These studies, the first to document functional adrenergic receptors at the nuclear membrane in any central nervous system cell, reveal a novel mechanism by which norepinephrine may directly influence nuclear processes. This mechanism may contribute to previously described neuroprotective, metabolic and immunoregulatory actions of norepinephrine.

Roles of Endomembrane Alkali Cation/Proton Exchangers in Synaptic Function and Neurodevelopmental Disorders

Endomembrane alkali cation (Na⁺, K⁺)/proton (H⁺) exchangers (eNHEs) are increasingly associated with neurological disorders. These eNHEs play integral roles in regulating the luminal pH, processing, and trafficking of cargo along the secretory (Golgi and post-Golgi vesicles) and endocytic (early, recycling, and late endosomes) pathways, essential regulatory processes vital for neuronal development and plasticity. Given the complex morphology and compartmentalization of multipolar neurons, the contribution of eNHEs in maintaining optimal pH homeostasis and cargo trafficking is especially significant during periods of structural and functional development and remodeling. While the importance of eNHEs has been demonstrated in a variety of non-neuronal cell types, their involvement in neuronal function is less well understood. In this review, we will discuss their emerging roles in excitatory synaptic function, particularly as it pertains to cellular learning and remodeling. We will also explore their connections to neurodevelopmental conditions, including intellectual disability, autism, and attention deficit hyperactivity disorders.

Natriuretic peptides are neuroprotective on in vitro models of PD and promote dopaminergic differentiation of hiPSCs-derived neurons via the Wnt/β-catenin signaling

Over the last 20 years, the efforts to develop new therapies for Parkinson's disease (PD) have focused not only on the improvement of symptomatic therapy for motor and non-motor symptoms but also on the discovering of the potential causes of PD, in order to develop disease-modifying treatments. The emerging role of dysregulation of the Wnt/β-catenin signaling in the onset and progression of PD, as well as of other neurodegenerative diseases (NDs), renders the targeting of this signaling an attractive therapeutic opportunity for curing this brain disorder. The natriuretic peptides (NPs) atrial natriuretic peptide (ANP), brain natriuretic peptide (BNP), and C-type natriuretic peptide (CNP), are cardiac and vascular-derived hormones also widely expressed in mammalian CNS, where they seem to participate in numerous brain functions including neural development/differentiation and neuroprotection. We recently demonstrated that ANP affects the Wnt/β-catenin pathway possibly through a Frizzled receptor-mediated mechanism and that it acts as a neuroprotective agent in in vitro models of PD by upregulating this signaling. Here we provide further evidence supporting the therapeutic potential of this class of natriuretic hormones. Specifically, we demonstrate that all the three natriuretic peptides are neuroprotective for SHSY5Y cells and primary cultures of DA neurons from mouse brain, subjected to neurotoxin insult with 6-hydroxydopamine (6-OHDA) for mimicking the neurodegeneration of PD, and these effects are associated with the activation of the Wnt/β-catenin pathway. Moreover, ANP, BNP, CNP are able to improve and accelerate the dopaminergic differentiation and maturation of hiPSCs-derived neural population obtained from two differed healthy donors, concomitantly affecting the canonical Wnt signaling. Our results support the relevance of exogenous ANP, BNP, and CNP as attractive molecules for both neuroprotection and neurorepair in PD, and more in general, in NDs for which aberrant Wnt signaling seems to be the leading pathogenetic mechanism.

Fructose and Mannose in Inborn Errors of Metabolism and Cancer

History suggests that tasteful properties of sugar have been domesticated as far back as 8000 BCE. With origins in New Guinea, the cultivation of sugar quickly spread over centuries of conquest and trade. The product, which quickly integrated into common foods and onto kitchen tables, is sucrose, which is made up of glucose and fructose dimers. While sugar is commonly associated with flavor, there is a myriad of biochemical properties that explain how sugars as biological molecules function in physiological contexts. Substantial research and reviews have been done on the role of glucose in disease. This review aims to describe the role of its isomers, fructose and mannose, in the context of inborn errors of metabolism and other metabolic diseases, such as cancer. While structurally similar, fructose and mannose give rise to very differing biochemical properties and understanding these differences will guide the development of more effective therapies for metabolic disease. We will discuss pathophysiology linked to perturbations in fructose and mannose metabolism, diagnostic tools, and treatment options of the diseases.

ASCT1 and ASCT2: Brother and Sister?

The SLC1 family includes seven members divided into two groups, namely, EAATs and ASCTs, that share similar 3D architecture; the first one includes high-affinity glutamate transporters, and the second one includes SLC1A4 and SLC1A5, known as ASCT1 and ASCT2, respectively, responsible for the traffic of neutral amino acids across the cell plasma membrane. The physiological role of ASCT1 and ASCT2 has been investigated over the years, revealing different properties in terms of substrate specificities, affinities, and regulation by physiological effectors and posttranslational modifications. Furthermore, ASCT1 and ASCT2 are involved in pathological conditions, such as neurodegenerative disorders and cancer. This has driven research in the pharmaceutical field aimed to find drugs able to target the two proteins.This review focuses on structural, functional, and regulatory aspects of ASCT1 and ASCT2, highlighting similarities and differences.

Bingeing on High-Fat Food Enhances Evoked Dopamine Release and Reduces Dopamine Uptake in the Nucleus Accumbens

OBJECTIVE

Binge-eating disorder (BED) disrupts dopamine neuron function, in part by altering dopamine transporter (DAT) activity. This study characterized the effects of high-fat bingeing on presynaptic dopamine terminals and tested the hypothesis that acute low-dose amphetamine would restore DAT function.

METHODS

C57BL/6 mice were given limited access (LimA) to a high-fat diet (2 h/d, 3 d/wk) or standard chow (control). After 6 weeks, ex vivo fast-scan cyclic voltammetry was used to characterize dopamine-terminal adaptations in the nucleus accumbens. Prior to undergoing fast-scan cyclic voltammetry, some mice from each group were given amphetamine (0.5 mg/kg intraperitoneally).

RESULTS

Escalation of high fat intake, termed bingeing, occurred in the LimA group and coincided with increased phasic dopamine release, reduced dopamine uptake rates, and increased dopamine receptor 2 (D2 ) autoreceptor function. Acute amphetamine selectively reversed dopamine uptake changes in the LimA group and restored the potency of amphetamine to inhibit uptake.

CONCLUSIONS

High-fat bingeing enhanced dopaminergic signaling in the nucleus accumbens by promoting phasic dopamine release and reducing clearance. This study's data show that amphetamine was efficacious in restoring impaired DAT function caused by high-fat bingeing but did not reduce dopamine release to normal. These presynaptic changes should be considered if amphetamine-like dopamine releasers are used as treatments for BED.

Glycosylation and raft endocytosis in cancer

Changes in glycosylation on proteins or lipids are one of the hallmarks of tumorigenesis. In many cases, it is still not understood how glycan information is translated into biological function. In this review, we discuss at the example of specific cancer-related glycoproteins how their endocytic uptake into eukaryotic cells is tuned by carbohydrate modifications. For this, we not only focus on overall uptake rates, but also illustrate how different uptake processes-dependent or not on the conventional clathrin machinery-are used under given glycosylation conditions. Furthermore, we discuss the role of certain sugar-binding proteins, termed galectins, to tune glycoprotein uptake by inducing their crosslinking into lattices, or by co-clustering them with glycolipids into raft-type membrane nanodomains from which the so-called clathrin-independent carriers (CLICs) are formed for glycoprotein internalization into cells. The latter process has been termed glycolipid-lectin (GL-Lect) hypothesis, which operates in a complementary manner to the clathrin pathway and galectin lattices.

Regulation of organic anion transporters: Role in physiology, pathophysiology, and drug elimination

The members of the organic anion transporter (OAT) family are mainly expressed in kidney, liver, placenta, intestine, and brain. These transporters play important roles in the disposition of clinical drugs, pesticides, signaling molecules, heavy metal conjugates, components of phytomedicines, and toxins, and therefore critical for maintaining systemic homeostasis. Alterations in the expression and function of OATs contribute to the intra- and inter-individual variability of the therapeutic efficacy and the toxicity of many drugs, and to many pathophysiological conditions. Consequently, the activity of these transporters must be highly regulated to carry out their normal functions. This review will present an update on the recent advance in understanding the cellular and molecular mechanisms underlying the regulation of renal OATs, emphasizing on the post-translational modification (PTM), the crosstalk among these PTMs, and the remote sensing and signaling network of OATs. Such knowledge will provide significant insights into the roles of these transporters in health and disease.

Effects of sleep disruption on stress, nigrostriatal markers, and behavior in a chronic/progressive MPTP male mouse model of parkinsonism

Sleep complaints are an early clinical symptom of neurodegenerative disorders. Patients with Parkinson's disease (PD) experience sleep disruption (SD). The objective of this study was to determine if preexisting, chronic SD leads to a greater loss of tyrosine hydroxylase (TH) within the striatum and the substantia nigra following chronic/progressive exposure with the neurotoxin, 1-methyl-2-phenyl-1,2,3,6-tetrahydropyridine (MPTP). Male mice underwent chronic SD for 4 weeks, then injected with vehicle (VEH) or increasing doses of MPTP for 4 weeks. There was a significant decrease in the plasma corticosterone levels in the MPTP group, an increase in the SD group, and a return to the VEH levels in the SD+MPTP group. Protein expression levels for TH in the striatum (terminals) and substantia nigra pars compacta (dopamine [DA] cell counts) revealed up to a 78% and 38% decrease, respectively, in the MPTP and SD+MPTP groups compared to their relevant VEH and SD groups. DA transporter protein expression increased in the striatum in the MPTP versus VEH group and in the SN/midbrain between the SD+MPTP and the VEH group. There was a main effect of MPTP on various gait measures (e.g., braking) relative to the SD or VEH groups. In the SD+MPTP group, there were no differences compared to the VEH group. Thus, SD, prior to administration of MPTP, has effects on serum corticosterone and gait but more importantly does not potentiate greater loss of TH within the nigrostriatal pathway compared to the MPTP group, suggesting that in PD patients with SD, there is no exacerbation of the DA cell loss.

A recurrent missense variant in SLC9A7 causes nonsyndromic X-linked intellectual disability with alteration of Golgi acidification and aberrant glycosylation

We report two unrelated families with multigenerational nonsyndromic intellectual disability (ID) segregating with a recurrent de novo missense variant (c.1543C>T:p.Leu515Phe) in the alkali cation/proton exchanger gene SLC9A7 (also commonly referred to as NHE7). SLC9A7 is located on human X chromosome at Xp11.3 and has not yet been associated with a human phenotype. The gene is widely transcribed, but especially abundant in brain, skeletal muscle and various secretory tissues. Within cells, SLC9A7 resides in the Golgi apparatus, with prominent enrichment in the trans-Golgi network (TGN) and post-Golgi vesicles. In transfected Chinese hamster ovary AP-1 cells, the Leu515Phe mutant protein was correctly targeted to the TGN/post-Golgi vesicles, but its N-linked oligosaccharide maturation as well as that of a co-transfected secretory membrane glycoprotein, vesicular stomatitis virus G (VSVG) glycoprotein, was reduced compared to cells co-expressing SLC9A7 wild-type and VSVG. This correlated with alkalinization of the TGN/post-Golgi compartments, suggestive of a gain-of-function. Membrane trafficking of glycosylation-deficient Leu515Phe and co-transfected VSVG to the cell surface, however, was relatively unaffected. Mass spectrometry analysis of patient sera also revealed an abnormal N-glycosylation profile for transferrin, a clinical diagnostic marker for congenital disorders of glycosylation. These data implicate a crucial role for SLC9A7 in the regulation of TGN/post-Golgi pH homeostasis and glycosylation of exported cargo, which may underlie the cellular pathophysiology and neurodevelopmental deficits associated with this particular nonsyndromic form of X-linked ID.

The SUMO-Conjugase Ubc9 Prevents the Degradation of the Dopamine Transporter, Enhancing Its Cell Surface Level and Dopamine Uptake

The dopamine transporter (DAT) is a plasma membrane protein responsible for the uptake of released dopamine back to the presynaptic terminal and ending dopamine neurotransmission. The DAT is the molecular target for cocaine and amphetamine as well as a number of pathological conditions including autism spectrum disorders, attention-deficit hyperactivity disorder (ADHD), dopamine transporter deficiency syndrome (DTDS), and Parkinson’s disease. The DAT uptake capacity is dependent on its level in the plasma membrane. In vitro studies show that DAT functional expression is regulated by a balance of endocytosis, recycling, and lysosomal degradation. However, recent reports suggest that DAT regulation by endocytosis in neurons is less significant than previously reported. Therefore, additional mechanisms appear to determine DAT steady-state level and functional expression in the neuronal plasma membrane. Here, we hypothesize that the ubiquitin-like protein small ubiquitin-like modifier 1 (SUMO1) increases the DAT steady-state level in the plasma membrane. In confocal microscopy, fluorescent resonance energy transfer (FRET), and Western blot analyses, we demonstrate that DAT is associated with SUMO1 in the rat dopaminergic N27 and DAT overexpressing Human Embryonic Kidney cells (HEK)-293 cells. The overexpression of SUMO1 and the Ubc9 SUMO-conjugase induces DAT SUMOylation, reduces DAT ubiquitination and degradation, enhancing DAT steady-state level. In addition, the Ubc9 knock-down by interference RNA (RNAi) increases DAT degradation and reduces DAT steady-state level. Remarkably, the Ubc9-mediated SUMOylation increases the expression of DAT in the plasma membrane and dopamine uptake capacity. Our results strongly suggest that SUMOylation is a novel mechanism that plays a central role in regulating DAT proteostasis, dopamine uptake, and dopamine signaling in neurons. For that reason, the SUMO pathway including SUMO1, SUMO2, Ubc9, and DAT SUMOylation, can be critical therapeutic targets in regulating DAT stability and dopamine clearance in health and pathological states.

The Human SLC1A5 (ASCT2) Amino Acid Transporter: From Function to Structure and Role in Cell Biology

SLC1A5, known as ASCT2, is a neutral amino acid transporter belonging to the SLC1 family and localized in the plasma membrane of several body districts. ASCT2 is an acronym standing for Alanine, Serine, Cysteine Transporter 2 even if the preferred substrate is the conditionally essential amino acid glutamine, with cysteine being a modulator and not a substrate. The studies around amino acid transport in cells and tissues began in the '60s by using radiolabeled compounds and competition assays. After identification of murine and human genes, the function of the coded protein has been studied in cell system and in proteoliposomes revealing that this transporter is a Na⁺ dependent antiporter of neutral amino acids, some of which are only inwardly transported and others are bi-directionally exchanged. The functional asymmetry merged with the kinetic asymmetry in line with the physiological role of amino acid pool harmonization. An intriguing function has been described for ASCT2 that is exploited as a receptor by a group of retroviruses to infect human cells. Interactions with scaffold proteins and post-translational modifications regulate ASCT2 stability, trafficking and transport activity. Two asparagine residues, namely N163 and N212, are the sites of glycosylation that is responsible for the definitive localization into the plasma membrane. ASCT2 expression increases in highly proliferative cells such as inflammatory and stem cells to fulfill the augmented glutamine demand. Interestingly, for the same reason, the expression of ASCT2 is greatly enhanced in many human cancers. This finding has generated interest in its candidacy as a pharmacological target for new anticancer drugs. The recently solved 3D structure of ASCT2 will aid in the rational design of such therapeutic compounds.

Linking Glycation and Glycosylation With Inflammation and Mitochondrial Dysfunction in Parkinson's Disease

Parkinson’s disease (PD) is the second most common neurodegenerative disorder, affecting about 6.3 million people worldwide. PD is characterized by the progressive degeneration of dopaminergic neurons in the Substantia nigra pars compacta, resulting into severe motor symptoms. The cellular mechanisms underlying dopaminergic cell death in PD are still not fully understood, but mitochondrial dysfunction, oxidative stress and inflammation are strongly implicated in the pathogenesis of both familial and sporadic PD cases. Aberrant post-translational modifications, namely glycation and glycosylation, together with age-dependent insufficient endogenous scavengers and quality control systems, lead to cellular overload of dysfunctional proteins. Such injuries accumulate with time and may lead to mitochondrial dysfunction and exacerbated inflammatory responses, culminating in neuronal cell death. Here, we will discuss how PD-linked protein mutations, aging, impaired quality control mechanisms and sugar metabolism lead to up-regulated abnormal post-translational modifications in proteins. Abnormal glycation and glycosylation seem to be more common than previously thought in PD and may underlie mitochondria-induced oxidative stress and inflammation in a feed-forward mechanism. Moreover, the stress-induced post-translational modifications that directly affect parkin and/or its substrates, deeply impairing its ability to regulate mitochondrial dynamics or to suppress inflammation will also be discussed. Together, these represent still unexplored deleterious mechanisms implicated in neurodegeneration in PD, which may be used for a more in-depth knowledge of the pathogenic mechanisms, or as biomarkers of the disease.

Identification and characterization of the Fasciola hepatica sodium- and chloride-dependent taurine transporter

The parasitic liver fluke Fasciola hepatica infests mainly ruminants, but it can also cause fasciolosis in people, who ingest the metacercariae encysted on plants. The drug of choice to treat fasciolosis is triclabendazole (TBZ), which has been on the market for several decades. This is also true for the other available drugs. Accordingly, drug-resistant flukes have been emerging at an increasing rate making it desirable to identify alternative drug targets. Here, we focused on the fact that adult F. hepatica persists in the hostile environment of the bile ducts of infected organisms. A common way to render bile acids less toxic is to conjugate them to taurine (2-aminoethanesulfonic acid). We cloned a transporter from the solute carrier-6 (SLC6) family, which was most closely related to the GABA-transporter-2 of other organisms. When heterologously expressed, this F. hepatica transporter supported the high-affinity cellular uptake of taurine (KM = 12.0 ± 0.5 μM) but not of GABA. Substrate uptake was dependent on Na+- and Cl- (calculated stoichiometry 2:1). Consistent with the low chloride concentration in mammalian bile, the F. hepatica transporter had a higher apparent affinity for Cl- (EC50 = 14±3 mM) than the human taurine transporter (EC50 = 55±7 mM). We incubated flukes with unconjugated bile acids in the presence and absence of taurine: taurine promoted survival of flukes; the taurine transporter inhibitor guanidinoethansulfonic acid abolished this protective effect of taurine. Based on these observations, we conclude that the taurine transporter is critical for the survival of liver flukes in the bile. Thus, the taurine transporter represents a candidate drug target.

Effects of taurine on striatal dopamine transporter expression and dopamine uptake in SHR rats

Dopaminergic deficits in the prefrontal cortex and striatum have been attributed to the pathogenesis of attention-deficit hyperactivity disorder (ADHD). Our recent study revealed that high-dose taurine improves hyperactive behavior and brain-functional signals in SHR rats. This study investigates the effect of taurine on the SHR striatum by detecting the spontaneous alternation, DA transporter (DAT) level, dopamine uptake and brain-derived neurotrophic factor (BDNF) expression. A significant increase in the total arm entries was detected in both WKY and SHR rats fed with low-dose taurine but not in those fed with high-dose taurine. Notably, significantly increased spontaneous alternation was observed in SHR rats fed with high-dose taurine. Significantly higher striatal DAT level was detected in WKY rats fed with low-dose taurine but not in SHR rats, whereas significantly reduced striatal DAT level was detected in SHR rats fed with high-dose taurine but not in WKY rats. Significantly increased dopamine uptake was detected in the striatal synaptosomes of both WKY and SHR rats fed with low-dose taurine. Conversely, significantly reduced dopamine uptake was detected in the striatal synaptosomes of SHR rats fed with high-dose taurine. Accordingly, a negative correlation was detected between striatal dopamine uptake and spontaneous alternation in SHR rats fed with low or high-dose taurine. Significantly increased BDNF was detected in the striatum of both WKY and SHR rats fed with low or high-dose taurine. These findings indicate that different dosages of taurine have opposite effects on striatal DAT expression and dopamine uptake, suggesting high-dose taurine as a possible candidate for ADHD treatment.

The dopamine transporter role in psychiatric phenotypes

The dopamine transporter (DAT) is one of the most relevant and investigated neurotransmitter transporters. DAT is a plasma membrane protein which plays a homeostatic role, controlling both extracellular and intracellular concentrations of dopamine (DA). Since unbalanced DA levels are known to be involved in numerous mental disorders, a wealth of investigations has provided valuable insights concerning DAT role into normal brain functioning and pathological processes. Briefly, this extensive but non-systematic review discusses what is recently known about the role of SLC6A3 gene which encodes the dopamine transporter in psychiatric phenotypes. DAT protein, SLC6A3 gene, animal models, neuropsychology, and neuroimaging investigations are also concisely discussed. To conclude, current challenges are reviewed in order to provide perspectives for future studies.

Role of intestinal trefoil factor in protecting intestinal epithelial cells from burn-induced injury

Although intestinal trefoil factor (ITF) can alleviate the burn-induced intestinal mucosa injury, the underlying mechanisms remains elusive. In this study, we investigated if ITF alters glutamine transport on the brush border membrane vesicles (BBMVs) of the intestines in Sprague-Dawley rats inflicted with 30% TBSA and the underlying mechanisms. We found that ITF significantly stimulated intestinal glutamine transport in burned rats. Mechanistically, ITF enhanced autophagy, reduces endoplasmic reticulum stress (ERS), and alleviates the impaired PDI, ASCT2, and B0AT1 in IECs and BBMVs after burn injury likely through AMPK activation. Therefore, ITF may protect intestinal epithelial cells from burn-induced injury through improving glutamine transport by alleviating ERS.

A direct interaction of cholesterol with the dopamine transporter prevents its out-to-inward transition

Monoamine transporters (MATs) carry out neurotransmitter reuptake from the synaptic cleft, a key step in neurotransmission, which is targeted in the treatment of neurological disorders. Cholesterol (CHOL), a major component of the synaptic plasma membrane, has been shown to exhibit a modulatory effect on MATs. Recent crystal structures of the dopamine transporter (DAT) revealed the presence of two conserved CHOL-like molecules, suggesting a functional protein-CHOL direct interaction. Here, we present extensive atomistic molecular dynamics (MD) simulations of DAT in an outward-facing conformation. In the absence of bound CHOL, DAT undergoes structural changes reflecting early events of dopamine transport: transition to an inward-facing conformation. In contrast, in the presence of bound CHOL, these conformational changes are inhibited, seemingly by an immobilization of the intracellular interface of transmembrane helix 1a and 5 by CHOL. We also provide evidence, from coarse grain MD simulations that the CHOL sites observed in the DAT crystal structures are preserved in all human monoamine transporters (dopamine, serotonin and norepinephrine), suggesting that our findings might extend to the entire family.

Functional properties of dopamine transporter oligomers after copper linking

Although it is universally accepted that dopamine transporters (DATs) exist in monomers, dimers and tetramers (i.e. dimers of dimers), it is not known whether the oligomeric organization of DAT is a prerequisite for its ability to take up dopamine (DA), or whether each DAT protomer, the subunit of quaternary structure, functions independently in terms of DA translocation. In this study, copper phenanthroline (CuP) was used to selectively target surface DAT: increasing concentrations of CuP gradually cross-linked natural DAT dimers in LLC-PK₁ cells stably expressing hDAT and thereby reduced DA uptake functionality until all surface DATs were inactivated. DATs that were not cross-linked by CuP showed normal DA uptake with DA K_m at ~ 0.5 μM and DA efflux with basal and amphetamine-induced DA efflux as much as control values. The cocaine analog 2β-carbomethoxy-3β-[4-fluorophenyl]-tropane (CFT) was capable to bind to copper-cross-linked DATs, albeit with an affinity more than fivefold decreased (K_d of CFT = 109 nM after cross-linking vs 19 nM before). A kinetic analysis is offered describing the changing amounts of dimers and monomers with increasing [CuP], allowing the estimation of dimer functional activity compared with a DAT monomer. Consonant with previous conclusions for serotonin transporter and NET that only one protomer of an oligomer is active at the time, the present data indicated a functional activity of the DAT dimer of 0.74 relative to a monomer.

Effect of the 5α-reductase enzyme inhibitor dutasteride in the brain of intact and parkinsonian mice

Dutasteride is a 5alpha-reductase inhibitor in clinical use to treat endocrine conditions. The present study investigated the neuroprotective mechanisms of action of dutasteride in intact and 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-lesioned mice using a low dose of MPTP not affecting motor activity modeling early stages of Parkinson's disease (PD). We hypothesized that dutasteride neuroprotection is due to altered steroids levels. Dutasteride pre-treatment prevented loss of striatal dopamine (DA) and its metabolite DOPAC. Dutasteride decreased effects of MPTP on striatal dopamine transporter (DAT), vesicular monoamine transporter 2 (VMAT2) and D2 DA receptor specific binding while D1 receptor specific binding remained unchanged. Dutasteride enhanced DAT specific binding and the glycosylated form of DAT in intact mice. MPTP-lesioned mice had plasma and brain testosterone and dihydrotestosterone levels lower than control mice whereas progesterone and its metabolites (dihydroprogesterone, isopregnanolone and tetrahydroprogesterone) pathway showed increases. Dutasteride treatment by inhibiting transformation of progesterone and testosterone to its metabolites elevated plasma and brain concentrations of testosterone compared to MPTP mice and decreased DHT levels in intact mice. Plasma and brain estradiol levels were low and remained unchanged by MPTP and/or dutasteride treatment. Dutasteride treatment did not affect striatal phosphorylation of Akt and its downstream substrate GSK3β as well as phosphorylation of ERK1/2 in intact and MPTP lesioned MPTP mice. Striatal glial fibrillary acidic protein (GFAP) levels were markedly elevated in MPTP compared to control mice and dutasteride reduced GFAP levels in MPTP mice. Treatment with dutasteride post-lesion left unchanged striatal DA levels. These results suggest dutasteride as promising drug for PD neuroprotection.

Pharmacochaperoning in a Drosophila model system rescues human dopamine transporter variants associated with infantile/juvenile parkinsonism

Point mutations in the gene encoding the human dopamine transporter (hDAT, SLC6A3) cause a syndrome of infantile/juvenile dystonia and parkinsonism. To unravel the molecular mechanism underlying these disorders and investigate possible pharmacological therapies, here we examined 13 disease-causing DAT mutants that were retained in the endoplasmic reticulum when heterologously expressed in HEK293 cells. In three of these mutants, i.e. hDAT-V158F, hDAT-G327R, and hDAT-L368Q, the folding deficit was remedied with the pharmacochaperone noribogaine or the heat shock protein 70 (HSP70) inhibitor pifithrin-μ such that endoplasmic reticulum export of and radioligand binding and substrate uptake by these DAT mutants were restored. In Drosophila melanogaster, DAT deficiency results in reduced sleep. We therefore exploited the power of targeted transgene expression of mutant hDAT in Drosophila to explore whether these hDAT mutants could also be pharmacologically rescued in an intact organism. Noribogaine or pifithrin-μ treatment supported hDAT delivery to the presynaptic terminals of dopaminergic neurons and restored sleep to normal length in DAT-deficient (fumin) Drosophila lines expressing hDAT-V158F or hDAT-G327R. In contrast, expression of hDAT-L368Q in the Drosophila DAT mutant background caused developmental lethality, indicating a toxic action not remedied by pharmacochaperoning. Our observations identified those mutations most likely amenable to pharmacological rescue in the affected children. In addition, our findings also highlight the challenges of translating insights from pharmacochaperoning in cell culture to the clinical situation. Because of the evolutionary conservation in dopaminergic neurotransmission between Drosophila and people, pharmacochaperoning of DAT in D. melanogaster may allow us to bridge that gap.

Exercise in an animal model of Parkinson's disease: Motor recovery but not restoration of the nigrostriatal pathway

Many clinical studies have reported on the benefits of exercise therapy in patients with Parkinson's disease (PD). Exercise cannot stop the progression of PD or facilitate the recovery of dopamine (DA) neurons in the substantia nigra pars compacta (SNpc) (Bega et al., 2014). To tease apart this paradox, we utilized a progressive MPTP (1-methyl-4-phenyl-1,2,3,6-tetra-hydropyridine) mouse model in which we initiated 4weeks of treadmill exercise after the completion of toxin administration (i.e., restoration). We found in our MPTP/exercise (MPTP+EX) group several measures of gait function that recovered compared to the MPTP only group. Although there was a small recovery of tyrosine hydroxylase (TH) positive DA neurons in the SNpc and terminals in the striatum, this increase was not statistically significant. These small changes in TH could not explain the improvement of motor function. The MPTP group had a significant 170% increase in the glycosylated/non-glycosylated dopamine transporter (DAT) and a 200% increase in microglial marker, IBA-1, in the striatum. The MPTP+EX group showed a nearly full recovery of these markers back to the vehicle levels. There was an increase in GLT-1 levels in the striatum due to exercise, with no change in striatal BDNF protein expression. Our data suggest that motor recovery was not prompted by any significant restoration of DA neurons or terminals, but rather the recovery of DAT and dampening the inflammatory response. Although exercise does not promote recovery of nigrostriatal DA, it should be used in conjunction with pharmaceutical methods for controlling PD symptoms.

Abnormal N-Glycosylation of a Novel Missense Creatine Transporter Mutant, G561R, Associated with Cerebral Creatine Deficiency Syndromes Alters Transporter Activity and Localization

Cerebral creatine deficiency syndromes (CCDSs) are caused by loss-of-function mutations in creatine transporter (CRT, SLC6A8), which transports creatine at the blood-brain barrier and into neurons of the central nervous system (CNS). This results in low cerebral creatine levels, and patients exhibit mental retardation, poor language skills and epilepsy. We identified a novel human CRT gene missense mutation (c.1681 G>C, G561R) in Japanese CCDSs patients. The purpose of the present study was to evaluate the reduction of creatine transport in G561R-mutant CRT-expressing 293 cells, and to clarify the mechanism of its functional attenuation. G561R-mutant CRT exhibited greatly reduced creatine transport activity compared to wild-type CRT (WT-CRT) when expressed in 293 cells. Also, the mutant protein is localized mainly in intracellular membrane fraction, while WT-CRT is localized in plasma membrane. Western blot analysis revealed a 68 kDa band of WT-CRT protein in plasma membrane fraction, while G561R-mutant CRT protein predominantly showed bands at 55, 110 and 165 kDa in crude membrane fraction. The bands of both WT-CRT and G561R-mutant CRT were shifted to 50 kDa by N-glycosidase treatment. Our results suggest that the functional impairment of G561R-mutant CRT was probably caused by incomplete N-linked glycosylation due to misfolding during protein maturation, leading to oligomer formation and changes of cellular localization.

Guidelines for Homology Modeling of Dopamine, Norepinephrine, and Serotonin Transporters

The human dopamine, norepinephrine, and serotonin transporters (hDAT, hNET, and hSERT) are carriers of neurotransmitters and targets for many drugs. Pioneering works in the past three years to elucidate experimental models of the Drosophila dDAT and human hSERT structures will rapidly impact the field of neuroscience. Here, we evaluated automated homology-based human models of these transporters, employing systematic physics-based, knowledge-based, and empirical-based check. Modeling guidelines were conveyed with attention to the central binding site (S1), secondary binding site (S2), and the extracellular loops EL2 and EL4. Application of new experimental models (dDAT and hSERT) will improve the accuracy of homology models, previously utilizing prokaryotic leucine transporter (LeuT) structure, and provide better predictions of ligand interactions, which is required for understanding of cellular mechanisms and for development of novel therapeutics.

N-Glycosylation influences transport, but not cellular trafficking, of a neuronal amino acid transporter SNAT1

SNAT1 is a system N/A neutral amino acid transporter that primarily expresses in neurons and mediates the transport of l-glutamine (Gln). Gln is an important amino acid involved in multiple cellular functions and also is a precursor for neurotransmitters, glutamate and GABA. In the present study, we demonstrated that SNAT1 is an N-glycoprotein expressed in neurons. We identified three glycosylation sites at asparagine residues 251, 257 and 310 in SNAT1 protein, and that the first two are the primary sites. The biotinylation and confocal immunofluorescence analysis showed that the glycosylation-impaired mutants and deglycosylated SNAT1 were equally capable of expressing on the cell surface. However, l-Gln and 3H-labeled methyl amino isobutyrate (MeAIB) was significantly compromised in N-glycosylation-impaired mutants and deglycosylated SNAT1 when compared with the wild-type control. Taken together, these results suggest that SNAT1 is an N-glycosylated protein with three de novo glycosylation sites and N-glycosylation of SNAT1 may play an important role in the transport of substrates across the cell membrane.

The glycosylation of the extracellular loop of β2 subunits diversifies functional phenotypes of BK Channels

Large-conductance Ca²⁺- and voltage-activated potassium (MaxiK or BK) channels are composed of a pore-forming α subunit (Slo) and 4 types of auxiliary β subunits or just a pore-forming α subunit. Although multiple N-linked glycosylation sites in the extracellular loop of β subunits have been identified, very little is known about how glycosylation influences the structure and function of BK channels. Using a combination of site-directed mutagenesis, western blot and patch-clamp recordings, we demonstrated that 3 sites in the extracellular loop of β2 subunit are N-glycosylated (N-X-T/S at N88, N96 and N119). Glycosylation of these sites strongly and differentially regulate gating kinetics, outward rectification, toxin sensitivity and physical association between the α and β2 subunits. We constructed a model and used molecular dynamics (MD) to simulate how the glycosylation facilitates the association of α/β2 subunits and modulates the dimension of the extracellular cavum above the pore of the channel, ultimately to modify biophysical and pharmacological properties of BK channels. Our results suggest that N-glycosylation of β2 subunits plays crucial roles in imparting functional heterogeneity of BK channels, and is potentially involved in the pathological phenotypes of carbohydrate metabolic diseases.